In a conventional electroacoustic transducer, in order to reproduce, with high quality, a wide band of a sound from a sound source as precisely as possible, various technical developments are in progress. Specifically, the bandwidth has been expanded from audio frequency to an ultrasonic band in order to reduce a sound conversion loss. In addition, various distortions in the band have been reduced. Further, an impulse response has been improved. Moreover, phase distortions, etc. have been adjusted.
For example, Japanese Laid Open Patent No. 2000-308177 discloses a ring shaped diaphragm having a V-character shape in a cross-sectional view thereof, taken along a circumferential direction of both ends of a cylindrical bobbin, on which a voice coil is winded, wherein both ends of the V-character shape are free. And further it discloses a speaker unit in which treble sound can be produced by piston vibration of the diaphragm, and super-treble sound can be produced in shape of a wave by differential vibration at the end portions of the diaphragm.
Moreover, Japanese Laid Open Patent No. H07-59186 discloses a method and apparatus for compensating a linear distortion of an acoustic signal in which linear distortion of the acoustic signal produced from a speaker system is compensated at a high frequency resolution in a low frequency band, and is compensated at a relatively low frequency resolution in a RF band since there is hearing property of the human beings who tends to feel a frequency change of an acoustic signal better as it is at a lower frequency, and it is hard to feel a frequency change at a higher frequency.
Furthermore, Japanese Laid Open Patent No. H11-252700 discloses that in an acoustic apparatus which uses a space in a car as a sound field, a phase property to the acoustic signal reproduced in the acoustic apparatus is adjusted.
The premise common to such technical developments is the mechanism of radiation of a sound wave in that sound is generated by dilatational wave (longitudinal wave) in which vibration is propagated in the air. In other words, the electroacoustic transducer in which mechanical vibration system inserted between an electrical signal and an acoustic signal (dilatational wave) is focused.
However, it is certain that vibrations generated in the nature are not limited to such dilatational wave (longitudinal wave). For example, in a familiar example, vibration of the bowstring of a violin is said to be a traverse wave.
On the other hand, the feeling of those who feel sound is called hearing or acoustic sense, in that an acoustic signal which enters in human's ears vibrates eardrums, and the vibration reaches a cerebrum center through a semicircular canal and a cochlear nerve. However, the mechanism of human's acoustic treatment capacity is still unknown in many fields.